Simulation of Thick Accretion Disks with Standing Shocks by Smoothed Particle Hydrodynamics

We present results of numerical simulation of inviscid thick accretion disks and wind flows around black holes. We use Smoothed Particle Hydrodynamics (SPH) technique for this purpose. Formation of thick disks are found to be preceded by shock waves travelling away from the centrifugal barrier. For a large range of the parameter space, the travelling shock settles at a distance close to the location obtained by a one-and-a-half dimensional model of inviscid accretion disks. Occasionally, it is observed that accretion processes are aided by the formation of oblique shock waves, particularly in the initial transient phase. The post-shock region (where infall velocity suddenly becomes very small) resembles that of the usual model of thick accretion disk discussed in the literature, though they have considerable turbulence. The flow subsequently becomes supersonic before falling into the black hole. In a large number of cases which we simulate, we find the formation of strong winds which are hot and subsonic when originated from the disk surface very close to the black hole but become supersonic within a few tens of the Schwarzschild radius of the blackhole. In the case of accretion of high angular momentum flow, very little amount of matter is accreted directly onto the black hole. Most of the matter is, however, first squeezed to a small volume close to the black hole, and subsequently expands and is expelled as a strong wind. It is quite possible that this expulsion of matter and the formation of cosmic radio jets is aided by the shock heating in the inner parts of the accretion disks.Comment: LaTeX, 16 pages, Astrophysical Journal (in press

Smoothed Particle Hydrodynamic Simulations of Viscous Accretion Discs Around Black Holes

Viscous Keplerian discs become sub-Keplerian close to a black hole since they pass through sonic points before entering into it. We study the time evolution of polytropic viscous accretion discs (both in one and two dimensional flows) using Smoothed Particle Hydrodynamics. We discover that for a large region of the parameter space, when the flow viscosity parameter is less than a critical value, standing shock waves are formed. If the viscosity is very high then the shock disappears. In the intermediate viscosity the disc oscillates very significantly in viscous time-scale. Our simulations indicate that these centrifugally supported high density region close to a black hole plays an active role in the flow dynamics, and consequently, the radiation dynamics.Comment: MNRAS style 6 pages of output, macros included. MNRAS (submitted

In-plane structure and ordering at liquid sodium surfaces and interfaces from ab initio molecular dynamics

Atoms at liquid metal surfaces are known to form layers parallel to the surface. We analyze the two-dimensional arrangement of atoms within such layers at the surface of liquid sodium, using ab initio molecular dynamics (MD) simulations based on density functional theory. Nearest neighbor distributions at the surface indicate mostly 5-fold coordination, though there are noticeable fractions of 4-fold and 6-fold coordinated atoms. Bond angle distributions suggest a movement toward the angles corresponding to a six-fold coordinated hexagonal arrangement of the atoms as the temperature is decreased towards the solidification point. We rationalize these results with a distorted hexagonal order at the surface, showing a mixture of regions of five and six-fold coordination. The liquid surface results are compared with classical MD simulations of the liquid surface, with similar effects appearing, and with ab initio MD simulations for a model solid-liquid interface, where a pronounced shift towards hexagonal ordering is observed as the temperature is lowered

Small values of signed harmonic sums

For every \u3c4 08R and every integer N, let mN(\u3c4) be the minimum of the distance of \u3c4 from the sums 11n=1Nsn/n, where s1,\u2026,sn 08 121,+1. We prove that mN(\u3c4

Minilaparoscopic Colorectal Resections: Technical Note

Laparoscopic colorectal resections have been shown to provide short-term advantages in terms of postoperative pain, general morbidity, recovery, and quality of life. To date, long-term results have been proved to be comparable to open surgery irrefutably only for colon cancer. Recently, new trends keep arising in the direction of minimal invasiveness to reduce surgical trauma after colorectal surgery in order to improve morbidity and cosmetic results. The few reports available in the literature on single-port technique show promising results. Natural orifices endoscopic techniques still have very limited application. We focused our efforts in standardising a minilaparoscopic technique (using 3 to 5 mm instruments) for colorectal resections since it can provide excellent cosmetic results without changing the laparoscopic approach significantly. Thus, there is no need for a new learning curve as minilaparoscopy maintains the principle of instrument triangulation. This determines an undoubted advantage in terms of feasibility and reproducibility of the procedure without increasing operative time. Some preliminary experiences confirm that minilaparoscopic colorectal surgery provides acceptable results, comparable to those reported for laparoscopic surgery with regard to operative time, morbidity, and hospital stay. Randomized controlled studies should be conducted to confirm these early encouraging results

The effect of cooling on time dependent behaviour of accretion flows around black holes

We present the results of several numerical simulations of two dimensional axi-symmetric accretion flows around black holes using Smoothed Particle Hydrodynamics (SPH) in the presence of cooling effects. We consider both stellar black holes and super-massive black holes. We observe that due to {\it both radial and vertical oscillation} of shock waves in the accretion flow, the luminosity and average thermal energy content of the inner disk exhibit very interesting behaviour. When power density spectra are taken, quasi-periodic variabilities are seen at a few Hz and also occasionally at hundreds of Hz for stellar black holes. For super-massive black holes, the time scale of the oscillations ranges from hours to weeks. The power density spectra have a flat top behavior with average {\it rms} amplitude of a few percent and a broken power-law behavior. The break frequency is generally found to be close to the frequency where the shock oscillates.Comment: 15 pages, 6 figures. Astronomy and Astrophysics (in press

Nucleosynthesis in Advective Accretion Disks Around Galactic and Extra-Galactic Black Holes

We compute the nucleosynthesis of materials inside advective disks around black holes. We show that composition of incoming matter can change significantly depending on the accretion rate and accretion disks. These works are improvements on the earlier works in thick accretion disks of Chakrabarti, Jin & Arnett (1987) in presence of advection in the flow.Comment: Latex pages including figures. Kluwer Style files included. Appearing in `Observational Evidence for Black Holes in the Universe', ed. Sandip K. Chakrabarti, Kluwer Academic Publishers (DORDRECHT: Holland

Trans-nasal endoscopic and intra-oral combined approach for odontogenic cysts

Maxillary cysts are a common finding in maxillofacial surgery, dentistry and otolaryngology. Treatment is surgical; a traditional approach includes Caldwell-Luc and other intra-oral approaches. In this article, we analyse the outcomes of 9 patients operated on using a combined intra-oral and trans-nasal approach to the aforementioned disease. Although the number of patients is small, the good results of this study suggest that the combined approach might be a reliable treatment option

Assessing the reliability and validity of Google Scholar indicators : The case of social sciences in Italy

Google Scholar is an appealing data source for the measurement of scientific production in the social sciences and humanities (SSH) fields. Its appeal derives from its extensive coverage of the literature. This contrasts with issues of data quality, which are still quite controversial. This chapter aims to evaluate the reliability and validity of bibliometric indicators taken from Scholar as well as their coverage of the scientific production in the social sciences. The analysis will be based on a comparison of Scholar with other bibliometric data sources (Web of Science and Scopus) plus an institutional dataset. The reliability of Scholar indicators will be investigated through correlational analysis, while their validity will be assessed using different external criteria (the results of national evaluation procedures based on a peer review approach). The analysis will be developed for the population of Italian university professors in a subset of SSH: political philosophy, history, political science and sociology. The final discussion of the results will take into account the various purposes that bibliometric exercises try to achieve